Reviewed by Alex SmithJul 7 2022
The current review has been conceived by academician Yunqi Liu and professor Yunlong Guo (Institute of Chemistry, Chinese Academy of Sciences). The co-first authors of the study are Dr. Zhiyuan Zhao, Dr. Kai Liu, and Yanwei Liu.
Technically, there are different routes for the realization of flexibility. 1) Physical flexibility: any rigid material that is extremely thin or has a very small diameter can be flexible. 2) Structural flexibility: for example, the wire-connecting fractal and spring configuration can provide macroscopic flexibility for the rigid chip. 3) Intrinsic flexibility: the materials in this device have flexible and stretchable nature. Different from the flexible and stretchable devices, the intrinsically flexible displays should simultaneously satisfy three critical requirements: large elastic deformation, small bending radius below 0.5 mm, and high stretching strain above 25%, which decides whether they can be subsequently conformed, folded, or rolled. With these three conditions, intrinsically flexible displays can change the perception of information that frequently appears in almost all aspects of human lives. Image Credit: ©Science China Press
This study fixes considerable attention to the main materials for electroluminescent devices and intrinsically flexible organic thin-film transistors (OTFTs).
The researchers have particularly focussed on the following five aspects: organic semiconductors (OSCs), intrinsically flexible electrode materials, dielectric materials for OTFTs, intrinsically flexible organic emissive semiconductors (OESCs) designed for electroluminescent devices, and OTFT-driven electroluminescent devices for intrinsically flexible displays.
The researchers also highlight the future challenges and opportunities regarding intrinsically expandable OTFT-driven displays.
Intrinsically flexible electrode materials should be defined by outstanding high mechanical stretchability, ideal adhesion, electrical conductivity, suitable work function, transparency, biocompatibility, and good chemical stability.
The authors offer an elaborate summary of present expandable electrode materials, such as graphene, metal nanowires (MNWs), carbon nanotubes (CNTs), hybrid materials, and their conducting polymers (CPs).
Intrinsically flexible organic semiconductors are an integral part of thin-film transistors. The present strategies are primarily split into the categories that are given: structurally designing polymer chains via the integration of conjugation-break spacers (CBs) and flexible chain segments, blending with elastomer polymers or molecular additives, and regulating molecular weight and regioregularity of conjugated polymers.
Intrinsically flexible dielectric material is close to the semiconductor layer and considerably impacts the transistors’ electrical performance. Currently, general elastomeric dielectric materials include PDMS, PU, and SEBS.
Normally, these elastomeric dielectrics display a low dielectric constant, thus raising the devices’ power consumption. This review suggested a few plans to develop high-k and high-stretchability dielectric polymers.
Intrinsically flexible organic light-emitting semiconductors are made just by introducing flexible chains present in the polymer matrix, so as to balance the mechanical submission and luminous capacities of organic light-emitting semiconductors.
Hence, exploiting a few new design methodologies to enhance the mechanical properties of materials is an essential direction for the growth of such materials.
Primarily, intrinsic flexible electroluminescent devices consist of organic light-emitting electrochemical cells (OLECs), stretchable alternating current electroluminescence (ACEL), and polymer light-emitting diodes (PLEDs).
As a result of the limitation by the properties of intrinsically stretchable electroluminescent materials, presently reported intrinsically flexible electroluminescent devices are primarily identified by the blending or doping methods.
Eventually, the authors propose a few recommendations and ideas for the future development of intrinsically flexible displays.
Journal Reference:
Zhao, Z., et al. (2022) Intrinsically flexible displays: key materials and devices. National Science Review. doi.org/10.1093/nsr/nwac090.